Lecture17A - (Free) Energies: Helmholtz and Gibbs(Chapter...

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Lecture 17 1 (Free) Energies: Helmholtz and Gibbs (Chapter 22.1,22.2) For an isolated system, dS>0 spontaneous process occurs. But how is an open beaker isolated? It isn’t, so we must consider ΔS of the surroundings as well. Then, ΔS universe = ΔS surr + ΔS sys >0 will determine spontaneity. This is a lot of work to determine if a process is spontaneous. Is there an easier way? Consider a real (not ideal!) system under isothermal conditions and at constant V: dU q w δ = + ( 29 ( 29 0 constant V because / ext w P dV q TdS dS q T = - = Since: and Then: so at constant V dU q TdS dU TdS = ( 29 0 at constant V 0 at constant T dU TdS d U TS - -
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2 Let's define Helmholtz (free) energy A U TS - = 0 dA where 0 at equilibrium and 0 for a spontaneous process dA dA = < The Helmholtz energy (based solely on the system) can tell us whether a process will occur spontaneously! The Helmholtz energy will continue to decrease for any spontaneous process until it reaches equilibrium, where A will be a minimum. Fig 22.1
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Lecture17A - (Free) Energies: Helmholtz and Gibbs(Chapter...

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